As the development of micro-machining techniques has progressed, microactuators have become increasingly important in various fields. For instance, optical switches that switch light paths used in optical communications, etc., may be cited as one example of a field in which microactuators are used. The optical switch disclosed in Japanese Patent Application Kokai No. 2001-142008 may be cited as one example of such an optical switch.
A microactuator comprises a fixed part and a movable part that is movable with respect to the fixed part; there are also microactuators of a type in which the movable part has a cantilever structure. For instance, an example in which the movable part has a cantilever structure is disclosed in FIG. 8 of Japanese Patent Application Kokai No. 2001-142008.
The optical switch disclosed in FIG. 8 of Japanese Patent Application Kokai No. 2001-142008 will be described below. This optical switch comprises an optical waveguide substrate on which optical waveguides are formed in the form of a matrix, and in which grooves into which mirrors can advance are formed at the intersection points of the matrix, and an actuator substrate on which microactuators and mirrors are formed. The mirrors are driven by the microactuators, and when the mirrors advance into the grooves of the optical waveguide substrate, the light is reflected by the mirrors, while when the mirrors are retracted from the grooves, the light advances directly forward, thus switching the light paths.
Furthermore, in the microactuator used in the optical switch disclosed in FIG. 8 of Japanese Patent Application Kokai No. 2001-142008, the movable part is formed only by a uniformly constructed plate spring part. A mirror is fastened to the tip end of this plate spring part. In a case where no driving force is applied, this plate spring part is bent toward the opposite side from the substrate, and when a driving force is applied, the entire surface of the plate spring part on the side of the substrate contacts the surface on the substrate. When the application of the driving force is stopped, the plate spring part returns to a state in which the plate spring part is bent toward the opposite side from the substrate as a result of the spring force (internal stress) of the plate spring part.
As a result of research conducted by the present inventor, it was ascertained that if the movable part is constructed only from a plate spring part that is uniformly constructed and that is bent toward the opposite side from the substrate in a state in which no force is received (as in the conventional microactuator described above) in cases where the movable part has a cantilever structure, it is difficult to operate the microactuator with a small driving force because of the construction of this movable part.
As one example, a case will be described in which a movable electrode is disposed on the tip end of the plate spring part and a fixed electrode is disposed on the substrate in a microactuator that has a movable part structure such as that of the conventional microactuator described above, and the electrostatic force that is generated between the two electrodes by the application of a voltage across the two electrodes is used as a driving force. In this case, in order to reduce the driving force that is required for operation, the distance between the two electrodes in a state in which no voltage is applied across the two electrodes may be shortened, and the length of the plate spring part may be lengthened so that the distance between the fixed end of the plate spring part and the movable electrode is increased. However, since the movable part is constructed only from a plate spring part that is uniformly constructed and that is bent toward the opposite side from the substrate in a state in which no force is received, it is impossible to accomplish both of these design elements at the same time. Specifically, the length of the plate spring part must be unavoidably shortened in order to shorten the distance between the two electrodes in a state in which no voltage is applied across the two electrodes. In this case, furthermore, the movement stroke of the mirrors cannot be sufficiently ensured. On the other hand, if the length of the plate spring part is increased, the distance between the two electrode parts is inevitably lengthened since the plate spring part is bent toward the opposite side from the substrate in a state in which no voltage is applied across the two electrodes. Accordingly, in a microactuator which has a movable part structure such as that of the conventional microactuator described above, it is difficult to operate the microactuator with a small driving force.